How fibre affects the body

The whole subject of food residue is a complicated one because of the variety of indigestible compounds and the different ways in which they influence the digestive and absorption processes in the body. In the simplest terms, dietary fibre acts in the digestive tract by binding or absorbing water and other substances, thus causing 'bulking' of the contents of the gut. It influences the speed with which food moves through the various parts of the digestive tract. However, the ways in which fibre affects digestion, absorption and the movement of food in the digestive tract are diverse, far-reaching and not always well understood. To illustrate this, fibre in the diet may affect blood sugar levels, the production of bile, the transport of cholesterol in the body and your chances of developing colon cancer. It can even affect the hormonal responses to a meal.

A variety of factors influence the effect of a particular fibre or high- fibre food. The chemical composition of the fibre, its particle size, the age of the plant source and methods used in the processing of the food, all affect the physical properties of the fibre. It is these characteristics which account for the different functions of fibre in addition to its main effect of causing in^creased bulk formation in the digestive tract.

Fibre effects are also influenced by the total composition of the meal and the amount of fibre in it. Short-term effects - for example on the blood sugar level - can be seen immediately after a person has eaten a single high-fibre meal. Long-term effects, such as lowering of blood cholesterol levels, occur only after a regular and prolonged intake of fibre. The effect of fibre in the different parts of the digestive tract (stomach, small and large intestine) also differs and may vary between individuals.

Fibre and the diseases of affluence

As already mentioned, research on dietary fibre was initially stimulated by studies which showed that population groups who follow high-fibre diets suffer far less from the 'diseases of affluence' - the medical term used to describe degenerative diseases and disorders which have been linked to a prosperous Western lifestyle. Among these are colon cancer, appendicitis, gall stones, varicose veins and constipation. Their causes are many; what has been established is that a variety of environmental factors, including dietary habits, can trigger these diseases in people who are genetically 'vulnerable' to them. This explains why they tend to run in families and why some people, even though they follow a Western lifestyle, do not develop a particular disease.

Let's look at those diseases which are known to be associated with the amount of fibre in our daily diets:

Constipation

Constipation is probably one of the most common disorders in the world and is also the cause of other diseases (see diverticular disease and colon cancer below.) It can be defined as the relatively slow movement of unduly firm waste matter through the large bowel, which results in the infrequent passing of small, hard and dry stools - often accompanied by straining.

Including enough fibre in your diet prevents constipation in various ways. Firstly, fibre is not digested in the small intestine. It therefore moves to the large bowel where it has a bulking effect, increasing the volume of the bowel content. Because it binds water, fibre prevents the absorption of water from the large bowel to some extent, ensuring that the contents remain large in volume and soft in consistency. Fibre also stimulates the growth of microbes in the large bowel, which contributes to larger, softer stools. The larger volume in the bowel stimulates peristaltic movements and eases the process of defecation so that straining is not necessary. In addition, during the partial fermentation of fibre in the large bowel by microbial enzymes, substances such as methane and hydrogen gasses, water and the short-chain fatty acids are produced. There is now convincing evidence that these acids play an important role in maintaining a healthy bowel.

Diverticular disease

It is now recognised that constipation - and the resultant increase in pressure in the colon (large bowel) because of strained bowel movements - is the underlying cause of diverticular disease. In this disorder, small, blown-out pouches or sac-like swellings (diverticula) form in the wall of the colon and project into the abdominal cavity. Inflammation of these pouches is known as diverticulitis and may cause abdominal discomfort and pain.

Diverticular disease is very common in affluent societies, especially among elderly people. The treatment of constipation with high-fibre diets will not only help prevent the development of the disease, but will also improve - and may even 'cure' - an existing condition.

Colon cancer

Although the role of diet in the development of this disease is still unclear, some scientists believe that high energy and fat and low fibre intakes are important contributing factors (see Cancer - can Your Diet Make the Difference?, page 181). The way in which fibre may protect against colon cancer is probably very complex. It is thought that the rapid movement of a bigger volume through the large bowel shortens the bowel wall's exposure time to substances which may induce cancer (carcinogens).

Because of its bulking and water-binding effects, fibre dilutes the concentration of carcinogens in the gut. It may even bind these substances and help with their excretion in the faeces. New research has showed that butyric acid, one of the short-chain fatty acids produced from fibre fermentation in the large bowel, also protects the bowel against the development of cancer. In addition, changes in colonic pH as a result of the fermentation process may be of importance.

Indifference - the biggest enemy? continue...

Colon cancer update

Some recent findings to do with the prevention of colon cancer (one of the most common cancers) are heartening. Studies on human beings have shown that including less fat and more fibre in one's dietprobably lessens the presence of `mutagens' (substances which can promote the development of cancer) in the bowel contents.

A recent four-year study in the USA also produced interesting results. The subjects of the research were a series of patients with 'familial adenomatous polyposis'. This refers to a segment of the population with bowel polyps which are particularly prone to become cancerous. (Polyps are growths with a pedicle or stem which usually develop on mucous membranes.) These patients were put on a high fibre diet (about 22g of fibre daily compared with the usual daily intake of about 12g), the extra fibre being provided by commercial bran cereal. In all the patients, a regression or shrinking of the polyps resulted - an outcome which was regarded as very promising by the US National Cancer Institute.

Also interesting is the fact that elderly blacks examined in the Department of Gastroenterology at Baragwanath Hospital, Soweto, showed no evidence of polyps; indeed their bowels had the appearance and elasticity of that of white teenagers! Among old rural blacks, colon cancer is extremely rare and even among urban dwellers, its occurrence is only a tenth of that found in elderly whites. The significance seems to be that the diets of these people are generallymuch higher in fibre and lower in fat than those of their white counterparts.

Eating to combat cancer - a day's sample menu**

* Breakfast: Cereals (such as All-Bran flakes or Weet-Bix), or oatmeal porridge, with milk and sugar

* Orange juice, a poached egg on brown (preferably wholewheat) toast and a couple of slices of tomato

Tea or coffee

* Lunch: 2 or 3 slices of brown or wholewheat bread with margarine,cottage cheese, salad (eg lettuce, celery, raw carrot), an apple or a banana

Tea or coffee

Dinner: Fish or chicken (not fried), brown rice, baked or newpotatoes, a green vegetable, baked beans, gem squash and a brown roll

Stewed or canned fruit (eg pineapple) and yoghurt

**This suggested menu provides about 30% of energy from fat and about 20-25g dietary fibre.

The alcohol/cancer link

Too much alcohol is a risk factor for the development of several cancers, especially of the liver, oesophagus, lung, pancreas and rectum. But here again, little notice has been taken of calls to cut down and levels of consumption are still rising, not only in many Western populations but in all Third World urban populations as well.

What is a reasonable or safe alcohol intake? For the majority of consumers there is no health problem, since it is considered that the majority of people who drink do so moderately and never experience any harmful effects. The crucial question is: What is a sensible limit? The liver burns up one standard drink (a half pint of beer, a glass ofwine, a measure of whisky) in an hour. With this in mind, the following safe allowances have been put forward:

For men, four to six standard drinks, two or three times a week. Or, at most, from 18 to 20 standard drinks weekly. For women, two or three standard drinks, two or three times a week or, at most, 9 to 13 standard drinks weekly.

These recommendations (endorsed by World Health 1985) were set out by the London Health Education Council in a booklet titled 'That's the Limit: a Guide to Sensible Drinking'. The reason why the limits for women are lower than for men is twofold: firstly, in men, water makes up 55-65°/o of body weight and in women, 45-55%, therefore alcohol is more diluted in men than in women; secondly, alcohol is potentially more noxious in women than in men.

Smoking and cancer

Smoking is believed to be responsible, directly or indirectly, for about a third of total cancers. It increases the risk of certain diet-related cancers such as breast cancer, but appears to have little influence with others, such as cancer of the prostate. In many Western populations there has been a major decrease in cigarette smoking among men, although far less so among women. In the USA, current frequencies are 32% for men, but 26% for women. Nowadays more young women than young men smoke, and today's female smoker is a heavier onethan yesterday's!

In Third World countries, tobacco use is rapidly increasing - in African populations, consumption has doubled in the last 10 years. So the dismal view is that all tobacco-related cancers, especially lung cancer, will almost certainly increase in the years to come.